1. Field of the Invention
The present invention relates to a liquid crystal display of an active matrix type, and more particularly to an active-matrix liquid crystal display of a multi-domain type with favorable viewing angle characteristics.
2. Description of the Prior Arts
Active-matrix liquid crystal displays which employ TN (twisted nematic) type liquid crystal have been widely used conventionally. An active-matrix liquid crystal display using the TN type liquid crystal comprises: a TFT (thin film transistor) substrate provided with a TFT for driving and a pixel electrode for each pixel; an opposite substrate having an opposite electrode thereon and disposed opposite to the TFT substrate; a color filter; and a polarizer. Liquid crystal is filled between the TFT substrate and the opposite substrate. In this liquid crystal display, when no voltage is applied between the pixel electrode and the opposite electrode, the TN liquid crystal molecules are aligned in parallel with the surface of the TFT substrate at that surface, and the alignment direction varies along the direction perpendicular to the substrate, resulting in twisted alignment of the liquid crystal. On the other hand, when a voltage is applied, the liquid crystal molecules between the substrates rise to change the polarization state in the liquid crystal layer. While the liquid crystal display using the TN type liquid crystal realizes selective display by utilizing the aforementioned change in the polarization state in the liquid crystal layer, visibility depends on the relationship between the alignment direction of the liquid crystal molecules and the position of a viewer, thereby presenting a problem of a small range of viewing angles for obtaining optimal visibility and resulting in insufficient viewing angle characteristics.
To overcome the problem, liquid crystal of a VA (vertical aligned) type has been proposed and put to practical use instead of the TN type liquid crystal for improving the insufficient viewing angle characteristics of the TN type liquid crystal display. A VA type liquid crystal display comprises a liquid crystal cell formed by liquid crystal in homeotropic (vertical) alignment with negative dielectric constant anisotropy between a TFT substrate and an opposite substrate, in which the liquid crystal molecules stand upright on the substrate when no voltage is applied, and when a voltage is applied, the liquid crystal molecules are laid in the substrate plane direction by an oblique electric field produced between a pixel electrode and an opposite electrode, thereby performing display. In this case, an area for one pixel is divided into a plurality of divided areas, and the directions in which the liquid crystal molecules are laid are varied among the divided areas to average the ways the display is viewed on the whole, which results in a wide viewing angle and favorable viewing angle characteristics. The divided area is also referred to as an alignment area. Such a liquid crystal display is referred to as a VA type multi-domain liquid crystal display.
While one pixel area is divided into a plurality of divided areas in accordance with directions in which the liquid crystal molecules are laid as described above in the VA type multi-domain liquid crystal display, it is important to provide stable dividing (partitioning) positions, that is, positions of boundaries between alignment areas. The dividing positions are boundaries of divided areas and the position in which so-called disclination occurs, and if division is made at an unexpected position, display quality is deteriorated.
As an attempt to stabilize the positions at which each pixel is divided into divided areas in the VA type multi-domain liquid crystal display, Japanese Patent Laid-open Publication No. 7-311383 (JP, A, 0731183) discloses a display which comprises an alignment control bank portion for each of a TFT substrate and an opposite substrate such that the positions at which the alignment control bank portions are provided are stabilized as dividing positions. FIG. 1 is a sectional view showing an example of a configuration of a VA type multi-domain liquid crystal display including an alignment control bank portion for each of a TFT substrate and an opposite substrate. FIG. 2 is a plan view schematically showing the arrangement of projections serving as the alignment control bank portions in the liquid crystal display shown in FIG. 1.
In the VA type multi-domain liquid crystal display shown in FIG. 1, liquid crystal layer 83 including a liquid crystal material with negative dielectric constant anisotropy is provided between TFT substrate 81 and opposite substrate 82 disposed opposite to each other. TFT substrate 81 comprises, on transparent support member 86, TFTs (thin film transistors) 87 for respective pixels and pixel electrodes 88 each electrically connected to a source electrode of corresponding TFT 87. On each pixel electrode 88, linear projections 89 each having a generally triangular cross section are provided as the alignment control bank portions. On the other hand, opposite substrate 82 has a structure in which color filter 92 and opposite electrode 93 are stacked in this order on transparent support member 91. On opposite electrode 93, linear projections 94 each having a generally triangular cross section are provided as the alignment control bank portions. As shown in FIG. 2, linear projections 89, 94 extend alternately in zigzag shape on TFT substrate 81 and opposite substrate 82, respectively.
Liquid crystal molecules 95 tend to be aligned perpendicularly to the surface of pixel electrode 88 and the surface of opposite electrode 93 when no voltage is applied. However, since the surfaces of projections 89 and 94 are inclined to the surfaces of pixel electrode 88 and opposite electrode 93, liquid crystal molecules 95 tend to be aligned in a direction slightly deviated from the direction perpendicular to the surfaces of pixel electrode 88 and opposite electrode 93. This affects the entire liquid crystal layer 83, and as a result, liquid crystal molecules 95 are aligned in a direction slightly deviated from the perpendicular direction even when no voltage is applied, as shown in FIG. 1. When a voltage is applied between pixel electrode 88 and opposite electrode 93, the alignment of liquid crystal molecules 95 is changed such that they are inclined in a direction deviated further from the perpendicular direction. The direction of the inclination when no voltage is applied depends on the positional relationships between liquid crystal molecules 95 and linear projections 89, 94 as shown in FIG. 2.
As a result, projections 89 and 94 determine the dividing position between divided areas A and B to stabilize the dividing position in this VA type multi-domain liquid crystal display.
In addition, Japanese Patent Laid-open Publication No. 8-76125 (JP, A, 08076125) proposes a display in which an opposite electrode itself is divided to stabilize dividing positions instead of using the aforementioned alignment control bank portions (i.e., linear projections). In a VA type liquid crystal display using a liquid crystal material with negative dielectric constant anisotropy, when a voltage is applied, the liquid crystal molecules tend to be aligned in the direction perpendicular to the electrical field. As shown in FIG. 3, division of opposite electrode 93 by slit 96 and pixel electrode 88 already provided for each pixel produce wraparound electric fields as shown in arrows in FIG. 3 between pixel electrode 88 and opposite electrode 93 when a voltage is applied. In the area in which the wraparound electric field is produced, i.e. near the ends of pixel electrode 88 and near the ends of opposite electrode 93, liquid crystal molecules 95 tend to be aligned perpendicularly to the direction of the wraparound electric field when a voltage is applied. For this reason, liquid crystal molecules 95 in the entire liquid crystal layer 83 are divided into a plurality of divided areas A and B in terms of alignment as shown in FIG. 3, and the boundary of the divided areas is formed in a stable position in accordance with the position of slit 96 and the shape or position of pixel electrode 88.
However, in an active-matrix liquid crystal display which serves as the aforementioned conventional VA type multi-domain liquid crystal display, an alignment control bank portion, if formed, must be provided for each of a TFT substrate and an opposite substrate. Since the opposite substrate need not be subjected to significant fine processing if no alignment control bank portion is provided, the provision of the alignment control bank portions not only for the TFT substrate but also for the opposite substrate requires fine processing on the opposite substrate, resulting in increased fabricating steps. Also, when the opposite electrode is divided by the slit, fine processing is required for the opposite electrode to cause increased fabricating steps.
It is an object of the present invention to provide an active-matrix liquid crystal display capable of stabilizing boundary positions of divided areas (alignment areas) without increasing fabricating steps, specifically, fine processing on an opposite substrate.
In the present invention for achieving the aforementioned object, a recess in groove shape is provided only for each pixel electrode as a division control structure without providing a particular division control structure for an opposite substrate.
In this case, if the pixel electrode is formed continuously across the recess, liquid crystal molecules within the recess are laid in the longitudinal direction of the recess when a voltage is applied between the pixel electrode and the opposite electrode, thereby fixing a boundary between divided areas.
Alternatively, in the case of a structure in which a conductive layer of a pixel electrode is removed in a recess, liquid crystal molecules in the recess remain perpendicular to both substrates when a voltage is applied, thereby fixing a boundary between divided areas.
In this manner, according to the present invention, a boundary position between divided areas (alignment areas) can be stabilized using the uniformly formed opposite electrode with the provision of the recess in groove shape only for the pixel electrode side and without requiring fine processing for alignment control on the opposite substrate side.
The above and other objects, features, and advantages of the present invention will become apparent from the following description referring to the accompanying drawings which illustrate examples of preferred embodiments of the present invention.